The MiMeS Project: Magnetism in Massive Stars

0.65 MB·

by  G. A. Wade

Cosmic Magnetic Fields: From Planets, to Stars and Galaxies
Proceedings IAU Symposium No. 259, 2009 (cid:13)c 2009International AstronomicalUnion
K.G. Strassmeier, A.G. Kosovichev & J.E. Beckman, eds. DOI:00.0000/X000000000000000X
The MiMeS Project:
Magnetism in Massive Stars
G.A. Wade1, E. Alecian1,2, D.A. Bohlender3, J.-C. Bouret4, J.H.
Grunhut1, H. Henrichs5, C. Neiner6, V. Petit7, N. St. Louis8, M.
Auri`ere9, O. Kochukhov10, J. Silvester1, A. ud-Doula11
and the MiMeS Collaboration†
1RoyalMilitary College of Canada, 2LESIA,France, 3Canadian Astronomy Data
Centre,4LAM, France,5Ast.Inst. Amsterdam, Netherlands, 6GEPI, France, 7Universit´e Laval,
Canada, 8Univ.deMontr´eal, Canada, 9LAT,France, 10Uppsala University,Sweden,
9 11Morrisville StateCollege, USA
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2 Abstract.TheMagnetisminMassiveStars(MiMeS)Projectisaconsensuscollaborationamong
the foremost international researchers of the physics of hot, massive stars, with the basic aim
n
of understanding the origin, evolution and impact of magnetic fields in these objects. The cor-
a
J nerstone of the project is the MiMeS Large Program at the Canada-France-Hawaii Telescope,
whichrepresentsadedicationof640hoursoftelescopetimefrom 2008-2012. TheMiMeS Large
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Program will exploit the unique capabilities of the ESPaDOnS spectropolarimeter to obtain

] critical missing information about the poorly-studied magnetic properties of these important
h stars, to confront current models and toguide theory.
p
- Keywords. Magnetic fields, massive stars, hot stars, star formation, stellar evolution, stellar
o
winds, spectropolarimetry
r
t
s
a
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1. Introduction
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v Massive stars are those stars with initial masses above about 8 times that of the sun,
8 eventually leading to catastrophic explosions in the form of supernovae.These represent
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the most massive and luminous stellar component of the Universe,and are the crucibles
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in which the lion’s share of the chemical elements are forged. These rapidly-evolving
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. stars drive the chemistry, structure and evolution of galaxies, dominating the ecology
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of the Universe - not only as supernovae, but also during their entire lifetimes - with
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far-reaching consequences.
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0 The magnetic fields of hot, higher-mass stars are qualitatively different from those
: of cool, low-mass stars (e.g. Wade 2003). They are detected in only a small fraction of
v
stars, and they are structurally much simpler, and frequently much stronger, than the
i
X fields of cool stars.Most remarkably,their characteristicsshow no clear correlationwith
r basicstellarpropertiessuchasage,massorrotation(e.g.Mathysetal.1997,Kochukhov
a
& Bagnulo 2006). The weight of opinion holds that these puzzling characteristics re-
flect a fundamentally different field origin: that the observed fields are not generated by
dynamos, but rather that they are fossil fields - the slowly-decaying remnants of field
accumulated or generated during star formation (e.g. Mestel 1999, Moss 2001, Ferrario
& Wickramasinghe 2006). This relic nature potentially provides us with a powerful and
unique capability: to study how magnetic fields evolve throughout the various stages of
stellarevolution,andtoexplorehowtheyinfluence,andareinfluencedby,theimportant
† www.physics.queensu.ca/∼wade/mimes
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120 G.A. Wade et al.
structural changes that occur during all phases of stellar evolution, from stellar birth to
stellar death.
Althoughthisfossilparadigmprovidesapowerfulframeworkforinterpretingthemag-
neticcharacteristicsofhigher-massstars,itsphysicaldetailsareonlyjustbeginningtobe
elaborated(e.g.Braithwaite&Nordlund2006,Auriereetal.2007,Alecianetal.2008a).
In particular, our knowledge of the basic statistical properties of massive star magnetic
fields is seriously incomplete. There is a troubling deficit in our understanding of the
scope of the influence of fields on massive star evolution, and almost no empirical basis
for how fields modify mass loss.
The Magnetism in Massive Stars (MiMeS) Project represents a comprehensive, mul-
tidisciplinary strategy by an international team of recognized researchersto address the
big questions relatedto the complex andpuzzling magnetismof massivestars.Recently,
MiMeSwasawarded”LargeProgram”statusbybothCanadaandFranceattheCanada-
France-Hawaii Telescope (CFHT), where the Project has been allocated 640 hours of
dedicated time with the ESPaDOnS spectropolarimeter from late 2008 through 2012.
This commitment of the observatory, its staff, its resources and expertise, allocated as
a result of an extensive international expert peer review of many competing proposals,
will be used to acquire an immense database of sensitive measurements of the optical
spectraandmagneticfieldsofmassivestars,whichwillbeappliedtoconstrainmodelsof
the origins of their magnetism, the structure, dynamics and emission properties of their
magnetospheres, and the influence of magnetic fields on stellar mass loss and rotation -
and ultimately the evolution of massive stars. More specifically, the scientific objectives
of the MiMeS Project are:
• To identify and model the physical processes responsible for the generationof mag-
netic fields in massive stars;
• To observe and model the detailed interaction between magnetic fields and massive
star winds;
• To investigate the role of the magnetic field in modifying the rotational properties
of massive stars;
• To investigate the impact of magnetic fields on massive star evolution,and the con-
nection between magnetic fields of non- degenerate massive stars and those of neutron
stars and magnetars, with consequential constraints on stellar evolution, supernova as-
trophysics and gamma-raybursts.
2. Structure of the Large Program
To address these general problems, we have devised a two-component Large Program
(LP)thatwillallowustoobtainbasicstatisticalinformationaboutthemagneticproper-
tiesofthe overallpopulationofhot,massivestars(the SurveyComponent),whilesimul-
taneously providing detailed information about the magnetic fields and related physics
of individual objects (the Targeted Component).
Targeted component: The MIMeS Targeted Component (TC) will provide data to
map the magnetic fields and investigate the physical characteristics of a small sample of
knownmagneticstarsofgreatinterest,atthehighestlevelofsophisticationpossible.The
roughly 20 TC targets have been selected to allow us to investigate a variety of physical
phenomena, and to allow us to directly and quantitatively confront models.
EachTCtargetwillbeobservedmanytimesusingtheESPaDOnSspectropolarimeter,
in order to obtain a high-precision and high-resolution sampling of the rotationally-
modulated circular and linear polarisation line profiles. Using state-of-the-art tomo-
graphic reconstruction techniques such as Magnetic Doppler Imaging (Kochukhov &
The MiMeS Project 121
Figure 1. Least-Squares Deconvolved profiles of 3 hot stars: θ1 Ori C (O7V, left), Par 1772
(B2V,middle)andNUOri(B0.5V,right).ThecurvesshowthemeanStokesI profiles(bottom
panel),themeanStokesV profiles(toppanel)andtheN diagnosticnullprofiles(middlepanel),
blackfor2006Januaryandredfor2007March.Eachstarexhibitsaclearmagneticsignaturein
StokesV.TheseresultsarerepresentativeofthoseexpectedfromtheMiMeSSurveyComponent.
From Petit et al. (2008).
Piskunov 2002), detailed maps of the vector magnetic field on and above the surface of
the star will be constructed.
Survey component: The MiMeS Survey Component (SC) will providecritical miss-
ing information about field incidence and statistical field properties for a much larger
sampleofmassivestars.Itwillalsoserveto providea broaderphysicalcontextfor inter-
pretationofthe results ofthe TargetedComponent.Froma much largerlist ofpotential
OB starscompiledfrompublishedcatalogues,we havegeneratedanSC sample ofabout
150targetswhichcoverthefullrangeofspectraltypesfromB2-O4whichareselectedto
be best-suited to field detection. Our target list includes pre-main sequence Herbig Be
stars, field and cluster OB stars, Be stars, and Wolf-Rayet stars.
Each SC target will be observed once or twice during the Project, at very high pre-
cision in circular polarisation. From the SC data we will measure the bulk incidence of
magneticmassivestars,estimatethevariationofincidenceversusmass,derivethestatis-
ticalproperties(intensityandgeometry)ofthemagneticfieldsofmassivestars,estimate
the dependence of incidence on age and environment, and derive the general statistical
relationshipsbetweenmagneticfieldcharacteristicsandX-rayemission,windproperties,
rotation, variability, binarity and surface chemistry diagnostics.
Of the 640 hours allocated to the MiMeS LP, 385 hours are committed to the SC and
255hoursarecommittedto the TC.The TC commitmentincludes 50hoursreservedfor
follow-up of targets detected in the Survey Component.
3. Precision magnetometry of massive stars
ForalltargetswewillexploitthelongitudinalZeemaneffectinmetalandheliumlines
todetectandmeasurephotosphericorpseudo-photosphericmagneticfields.Splittingofa
spectrallinedue toalongitudinalmagneticfieldintooppositely-polarizedσ components
produces a variation of circular polarisation across the line (commonly referred to as
a (Stokes V) Zeeman signature or magnetic signature; see Fig. 1.). The amplitude and
morphologyoftheZeemansignatureencodeinformationaboutthestrengthandstructure
of the global magnetic field. For some TC targets, we will also exploit the transverse
Zeemaneffectto constrainthe detailedlocalstructureofthe field.Splitting ofa spectral
linebyatransversemagneticfieldintooppositely-polarizedπandσcomponentsproduces
avariationoflinearpolarisation(characterizedbytheStokesQandU parameters)across
the line (e.g. Kochukhov et al. 2004).
122 G.A. Wade et al.
Figure 2. Magnetic Doppler Imaging (MDI) of the B9p star HD 112413 (Kochukhovet al., in
preparation),illustratingthereconstructedmagneticfieldorientation (lowerimages) andinten-
sity(upperimages)ofthisstarat5rotationphases.Themapswereobtainedfromatime-series
of 21 Stokes IQUV spectral sequences. Although the field line orientation of HD 112413 is ap-
proximately dipolar, the field intensity map is far more complex. Maps similar to these will be
constructed for theMiMeS Targeted Component.
3.1. Survey Component
For the SC targets, the detection of magnetic field is diagnosed using the Stokes V
detectioncriteriondescribedbyDonatietal.(1992,1997),andthesurfacefieldconstraint
characterised using the powerful Bayesian estimation technique of Petit et al. (2008).
After reduction of the polarized spectra using the Libre-Esprit optimal extraction code,
weemploytheLeast-SquaresDeconvolution(LSD;Donatietal.1997)multi-lineanalysis
procedure to combine the Stokes V Zeeman signatures from many spectral lines into a
singlehigh-S/Nmeanprofile(seeFig.1),enhancingourabilitytodetectsubtlemagnetic
signatures. Least-Squares Deconvolution of a spectrum requires a line mask to describe
the positions,relativestrengthsandmagneticsensitivitiesofthe lines predictedto occur
in the stellar spectrum. The line mask characteristics are sensitive to the parameters
describingthe stellaratmosphere.Inouranalysisweemploycustomline maskscarefully
tailoredtobestreproducetheobservatedstellarspectrum,inordertomaximizetheS/N
gain of the LSD procedure and therefore our sensitivity to weak magnetic fields.
TheexposuredurationrequiredtodetectaZeemansignatureofagivenstrengthvaries
asafunctionofstellarapparentmagnitude,spectraltypeandprojectedrotationalveloc-
ity.Thisresultsinalargerangeofdetectionsensitivitiesforourtargets.TheSCexposure
timesarebasedonanempiricalexposuretimerelationderivedfromrealESPaDOnSob-
servations of OB stars, and takes into account detection sensitivity gains resulting from
LSD and velocity binning, and sensitivity losses from line broadening due to rapid ro-
tation. Exposure times for our SC targets correspond to the time required to definitely
detect (with a false alarm probability below 10−5) the Stokes V Zeeman signature pro-
duced by a surface dipole magnetic field with a specified polar intensity. Although our
calculated exposure times correspond to definite detections of a dipole magnetic field,
our observations are also sensitive to the presence of substantially more complex field
toplogies.
3.2. Targeted Component
ZeemansignatureswillbedetectedrepeatedlyinallspectraofTCtargets.Thespectropo-
larimetric timeseries will be interpreted using several magnetic field modeling codes at
The MiMeS Project 123
Figure 3. Example of the spectral and spatial emission properties of a rotating massive star
magnetosphere modeled using Rigid Field Hydrodynamics (Townsend et al. 2007). The stel-
lar rotation axis (vertical arrow) is oblique to the magnetic axis (inclined arrow), leading to a
complex potential field produced by radiative acceleration, Lorentz forces and centripetal ac-
celeration. Theconsequentheatedplasma distributionin thestellar magnetosphere(illustrated
in colour/grey scale) shows broadband emission, and is highly structured both spatially and
spectrally. Magnetically-confined winds such as this are responsible for theX-ray emission and
variability properties of some OB stars, and models such as this will be constructed for the
MiMeS Targeted Component.
our disposal. For those stars for which Stokes V LSD profiles will be the primary model
basis, the modeling codes employed by Donati et al. (2006) or Alecian et al. (2008b)
will be employed. For those stars for which the signal-to-noise ratio in individual spec-
tral lines is sufficient to model the polarisation spectrum directly, we will employ the
Invers10 Magnetic Doppler Imaging code to simultaneously model the magnetic field,
surface abundance structures and pulsation velocity field (Piskunov & Kochukhov 2002,
Kochukhov et al. 2004). The resultant magnetic field models will be compared directly
with the predictions of fossil and dynamo models (e.g. Braithwaite 2006, 2007, Mullan
& Macdonald 2005, Arlt 2008).
Diagnosticsofthewindandmagnetosphere(e.g.opticalemissionlinesandtheirlinear
polarisation,UV line profiles, X-ray photometry and spectroscopy,radio flux variations,
etc.) will be modeled using both the semi-analytic Rigidly-Rotating Magnetosphere ap-
proach, the Rigid-Field Hydrodynamics (Townsend et al. 2007) approach and full MHD
simulations using the 3D ZEUS code (e.g. Stone & Norman 1992; see Fig. 3).
4. MiMeS data pipeline
Following their acquisition in Queued Service Observing mode at the CFHT, ES-
PaDOnS polarised spectra are immediately reduced by CFHT staff using the Libre-
Esprit reduction package and downloaded to the dedicated MiMeS Data Archive at the
Canadian Astronomy Data Centre in Victoria, Canada. Reduced spectra are carefully
normalizedto the continuumusing existingsoftwaretailoredto hotstellarspectra.Each
reduced ESPaDOnS spectrum is then subject to an immediate quick-look analysis to
verify nominal resolving power, polarimetric performance and S/N. Preliminary LSD
profiles are extracted using our database of generic hot star line masks to perform an
initial magnetic field diagnosis and further quality assurance. Finally, each ESPaDOnS
spectrum will be processed by the MiMeS Massive Stars Pipeline (MSP; currently in
production) to determine a variety of critical physical data for each observed target, in
addition to the precision magnetic field diagnosis:effective temperature, surface gravity,
mass,radius,age,variabilitycharacteristics,projectedrotationalvelocity,radialvelocity
124 G.A. Wade et al.
andbinarity,andmasslossrate.Thesemeta-data,inadditiontothereducedhigh-quality
spectra, will be uploaded for publication to the MagIcS Legacy Database†.
5. First results
MiMeS operations beganin August 2008,andnearly one semester ofobservations has
been acquired at the time of writing. This corresponds to approximately 70 hours of
observation, during which more than 200 polarised spectra were acquired for about 50
MiMeStargets.Furtherdetailsaboutthe firstresultsoftheMiMeSProjectarereported
by Grunhut et al. (these proceedings).
Acknowledgements
The MiMeS Large Programis supported by both Canada and France, and was one of
4 such programs selected in early 2008 as a result of an extensive international expert
peer review of many competing proposals.
BasedonobservationsobtainedattheCanada-France-HawaiiTelescope(CFHT)which
is operated by the National Research Council of Canada, the Institut National des Sci-
encesde l’Univers ofthe CentreNationalde la RechercheScientifique ofFrance,andthe
University of Hawaii.
The MiMeSDataAccess Pagesarepoweredby softwaredevelopedby the CADC,and
contains data and meta-data provided by the CFHT Telescope.
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† The MiMeS Project is undertaken within the context of the broader
MagIcS (Magnetic Investigations of various Classes of Stars) collaboration,
www.ast.obs-mip.fr/users/donati/magics).